Heterotic and M-theory Compactifications for String Phenomenology

In this thesis, we explore two approaches to string phenomenology. In the first half of the work, we investigate M-theory compactifications on spaces with co-dimension four, orbifold singularities. We construct M-theory on C^2/Z_N by coupling 11-dimensional supergravity to a seven-dimensional Yang-Mills theory located on the orbifold fixed-plane. The resulting action is supersymmetric to leading non-trivial order in the 11-dim Newton constant. We thereby reduce M-theory on a G2 orbifold with C^2/Z_N singularities, explicitly incorporating the additional gauge fields at the singularities. We derive the Kahler potential, gauge-kinetic function and superpotential for the resulting N=1 four-dimensional theory. Blowing-up of the orbifold is described by a Higgs effect and the results are consistent with the corresponding ones obtained for smooth G2 spaces. Further, we consider flux and Wilson lines on singular loci of the G2 space, and discuss the relation to N=4 SYM theory. In the second half, we develop an algorithmic framework for E8 x E8 heterotic compactifications with monad bundles. We begin by considering cyclic Calabi-Yau manifolds where we classify positive monad bundles, prove stability, and compute the complete particle spectrum for all bundles. Next, we generalize the construction to bundles on complete intersection Calabi-Yau manifolds. We show that the class of positive monad bundles, subject to the heterotic anomaly condition, is finite (~7000 models). We compute the particle spectrum for these models and develop new techniques for computing the cohomology of line bundles. There are no anti-generations of particles and the spectrum is manifestly moduli-dependent. We further study the slope-stability of positive monad bundles and develop a new method for proving stability of SU(n) vector bundles.